Fiber optic cable is a common type of cabling used to transmit optical data from one location to another. Fiber optic cable is generally comprised of either glass, a combination of glass and polymers, or just polymers (plastic optical fibers). Fiber optic cable is fabricated in such a way that it can conduct a beam of light from one end of the cable to another.
A typical fiber optic cable 10 is illustrated in FIG. 1. As seen in FIG. 1, fiber optic cable 10 is generally comprised of a core 12, cladding 14, and buffer/outer jacket 16. The core 12 is a very narrow strand of high quality glass and is carried through the cable 10 by way of the cladding 14. The cladding 14 is also made of high quality glass but has a slightly lower index of refraction than the core, usually within 1-2%. Thus, if the light injected into the core 12 strikes cladding 14 the light is reflected back into the core 12 so as to continue down the cable 10. The jacket 16 acts as a shock absorber to protect core 12 and cladding 14 from shocks that might affect their physical properties. Further, jacket 16 protects the cable 10 from abrasions, solvents, and other contaminants. Jacket 14 does not have any optical properties that might affect the propagation of light within the cable 10.
A typical prior art fiber optic data link is illustrated generally at 18 in FIG. 2. The data link 18 generally comprises a source 20, a transmitter 22, one or more fiber optic cables 10, a receiver 26, and an end user 28. Source 20 provides data to the transmitter 22 in the form a digital electrical signal. The transmitter 22 acts as a transducer and converts the digital electrical signal into an optical signal. The transmitter 22 comprises a light source for transmitting the optical signal through the fiber optic cable 10. The transmitter 22 modulates the light so as to represent the binary data it receives from source 20. The receiver 26 has two functions. First, receiver 26 senses or detects light from the fiber optic cable 10 and then converts the light into an electrical signal. Second, receiver 26 demodulates this light to determine the data that it represents. The receiver 26 then transmits the binary data to the user in the form of an electrical signal.
The fiber optic cable 10 is mated to the transmitter 22 and receiver 26 by connectors 30. Each connector 30 is comprised of a main body 32, a ferrule 34, and an aperture 36. At the terminus of cable 10, all layers of cable 10 are stripped away except for core 12, cladding 14, and sometimes the protective buffer coating 16. The cable 10 is then inserted within aperture 36 of main body 32 until the stripped end of the cable 10 extends through ferrule 34.
Further, as seen in FIG. 3, the connectors 30 of one or more cables 10 may be linked so as to increase the distance between transmitter 22 and receiver 26. The mating of each connector 30 is provided by adaptor housing 38. Adaptor housing 38 is comprised of a first half 40 and a second half 42. The halves 40, 42 each have apertures 44 to secure each connector 30 to opposite sides of adaptor 38.
To provide optimum optical transmission performance between connectors 30 when two or more cables 10 are linked, the cores 12 of each cable 10 must be precisely aligned co-axially. To aid in the co-axial alignment of the cores 12, the ferrules 34 are typically placed within alignment sleeve 46. Alignment sleeve 46 is a cylinder-like metal or ceramic device that mechanically clasps an outside diameter of the ferrules 34 to bring the ferrules 34 into co-axial alignment.
Precise co-axial alignment of cores 12 using housing 38 and alignment sleeve 46 can only be achieved if the cores 12 of both cables 10 are precisely centered within each ferrule 34. If the cores 12 are not centered then the cores 12 will not be aligned even if the ferrules 34 are aligned and optical transmission loss is experienced as light is unable to travel uninterrupted between connectors 30.
In a lab setting, precise co-axial alignment of cores 12 is easily achieved. Specifically, the cables 10 are disconnected from transmitter 22 and receiver 26 to allow a continuous wave of light to be inserted through the cables 10. The light output is measured by a suitable optical power meter as it passes through the connectors 30 associated with adaptor housing 38. As the output is measured, the cable 10 or cable/ferrule 10/34 is rotated. When the highest level of optical power is recorded by the power meter, rotation is stopped and the positions of the cables 10 are locked in place using any suitable device, such as a locking connector.
The above described technique for determining the precise co-axial alignment of the cores 12 requires that connectors 30 of cables 10 be removed from transmitter 22 and receiver 26. Consequently the method is only suitable for laboratory use and not for field use because removing cables 10 will likely cause the cables 10 to be damaged due to the infiltration of foreign materials. Specifically, in stressful repair scenarios, such as on an aircraft carrier deck, the cables 10 may be damaged by salt spray, grease, or other substances harmful to optical fibers.
Thus, there exists a need for a method capable of determining the precise co-axial alignment of fiber optic cores 12, and maximum optical transmission performance, without having to disconnect the fiber optic cables 10 and expose the connections 30 to the atmosphere, thus risking performance degradation due to the infiltration of foreign elements, such as dust and dirt.